Thermal generator and combustion method for limiting nitrogen oxides emissions by re-combustion of fumes

ABSTRACT

The invention relates to a thermal generator comprising a furnace tube ( 2 ) wherein a fuel is burnt, recombustion means ( 14, 15 ) for reducing the nitrogen oxides content present in said fumes and means ( 3 ) for recovering the heat of the fumes resulting from said combustion. The invention is characterized in that recombustion means ( 14, 15 ) are arranged in containment means ( 11 ).

FIELD OF THE INVENTION

[0001] The present invention relates to the sphere of thermalgenerators, notably industrial boilers, allowing to limit nitrogenoxides emissions by recombustion of the fumes and to a method forimplementing such a generator.

[0002] The invention is applicable to any type of boiler such as, forexample, fume-tube boilers, water-tube boilers, ambitubular boilers, andit can be implemented whatever the thermal power of said boiler.

BACKGROUND OF THE INVENTION

[0003] There are well-known methods for recombustion of the fumesdischarged so as to reduce the NO_(x) emissions, such as those describedin patents U.S. Pat. No. 5,139,755 and WO-97/25,134.

[0004] This recombustion is a nitrogen oxides reduction technique basedon stepping of the combustion. A furnace usually contained in theseboilers, wherein this technique is used, comprises three zones:

[0005] the first zone wherein about 85 to 95% by mass of the fuel isburnt under standard conditions, i.e. with about 5 to 15% excess airwhen gaseous fuels or liquid fuels are used;

[0006] in the second zone, downstream from the first zone, the remainingfuel which consumes the excess oxygen of the fumes from the first zoneis injected. The atmosphere of this second zone becomes reducing and thenitrogen oxides generated in the first zone are essentially converted tomolecular nitrogen under the action of hydrocarbon-containing radicals;

[0007] in a third zone (or postcombustion zone), air is added so as toeliminate all the unburnt substances generated in the second zone and tohave a standard excess air of 5 to 15% at the outlet.

[0008] The recombustion method described above generally allows todecrease the NO_(x) emissions by about 50 to 80%.

[0009] Although in its principle recombustion is attractive as regardsperformances and insofar as it requires no reactant allowing to reducethe nitrogen oxides other than the fuel itself, it however presentsconsiderable drawbacks.

[0010] In the case of a thermal generator working with natural gas, themost significant difficulties encountered are:

[0011] high corrosion in the recombustion zone and in the postcombustionzone, due to the presence of a reducing atmosphere or to alternatingoxidizing and reducing atmospheres,

[0012] imperfect oxidation of the recombustion fuel in thepostcombustion zone and, as a consequence, formation of gaseous andsolid unburnt residues and, in the most severe cases, fouling of thedownstream exchange surfaces, which reduces the overall energyefficiency and requires more sophisticated and expensive automaticcleaning equipments,

[0013] safety difficult to provide because of the stage of injection ofa fuel in the recombustion zone. The main risk is a non-combustion ofsaid fuel owing to an operating trouble or to an ill-controlledtransient operation. In this case, there are risks of explosion in theparts situated downstream from the recombustion zone (recovery boiler,filter, etc.).

[0014] In the case of a thermal generator working with heavy petroleumproducts, the implementation difficulties are the same as thoseencountered with natural gas, but they are often increased for some ofthem. This is in particular the case for solid unburnt residues producedin larger quantities. Furthermore, the denitrification efficiency can belower with these heavy products because of the presence ofnitrogen-containing compounds in the initial fuel.

SUMMARY OF THE INVENTION

[0015] The present invention aims to overcome the aforementioneddrawbacks by means of a thermal generator comprising a furnace tubewherein a fuel is burnt, recombustion means allowing to reduce thenitrogen oxides content present in said fumes, and means for recoveringthe heat of the fumes from said combustion, characterized in that itcomprises containment means wherein said recombustion means arearranged.

[0016] Advantageously, the containment means are arranged between thefurnace tube and the heat recovery means.

[0017] Said containment means can be extractable and fastened to a trapdoor.

[0018] Preferably, the containment means can include means allowing tomodify the velocity profile of the fumes at the inlet of saidcontainment means.

[0019] Advantageously, a fume box can be arranged between thecontainment means and said heat recovery means.

[0020] Said fume box can comprise tubes for heat exchange with thefumes.

[0021] Preferably, said containment means can include a shell whereinmeans for injecting a recombustion fuel and means for generating a pilotflame are successively arranged, in the direction of circulation of theflames.

[0022] Said containment means can also comprise means for injecting airintended for postcombustion.

[0023] The pilot flame can be positioned substantially halfway betweenthe postcombustion air injection point and the recombustion fuelinjection point.

[0024] Besides, means for injecting air intended for postcombustion canbe arranged downstream from said shell.

[0025] The invention also relates to a method for limiting nitrogenoxides emissions discharged by a thermal generator, characterized inthat the following stages are carried out:

[0026] a) burning a major part of the fuel in a combustion zone,

[0027] b) passing the fumes resulting from said combustion into acontainment zone (11),

[0028] c) mixing in said containment zone a minor part of the fuel withsaid fumes.

[0029] According to a preferred embodiment, air can be injected into thefumes resulting from stage c) during a stage d).,

[0030] According to another variant, stage d) can be carried out in saidcontainment zone.

[0031] According to another embodiment, about 70% to about 95% of thetotal fuel mass can be burnt during stage a), about 5% to about 30% ofthe total fuel mass can be burnt during stage c) and an amount of airallowing to have excess air in relation to stoichiometric conditions ofabout 5% to about 25% can be injected during stage d).

[0032] The fumes from stage c) can be contacted with a pilot flame priorto stage d).

[0033] A chemical reactant other than the fuel can be added to the airinjected during stage d) to allow reduction of the nitrogen oxides byselective non-catalytic means.

[0034] According to an advantageous embodiment, when the generator worksunder reduced operating conditions, the flow rates of the fuel injectedin the combustion zone and in the containment zone can be adjusted so asto maintain a substantially constant temperature in said containmentzone.

[0035] The invention thus provides a simple, efficient and safe solutionfor recombustion of boiler fumes allowing to significantly reduceemissions of pollutants and in particular of nitrogen oxides (NO_(x)).

[0036] The device and/or the method according to the inventionadvantageously allows to reduce nitrogen oxides discharges by 30% to90%, preferably by 50% to 75% in industrial boilers whose powergenerally ranges between 100 kWth (thermal kilowatts) and 50 MWth(thermal megawatts) without requiring costly and sophisticatedequipments since they are limited to a shell and to fuel and oxidizerinjection means.

[0037] Furthermore, this mode of reducing nitrogen oxides emissions doesnot require other <<reactants>> than the fuels used by the boiler.

[0038] The solution provided is all the more interesting as it is oftendifficult, or even impossible, to install known specific burners thatproduce only small amounts of nitrogen oxides (also referred to aslow-NO_(x) burners) in confined furnaces such as those encountered infume-tube boilers or flash boilers. The solution provided by the presentinvention can also be applied to existing boilers, with minor changesbrought to the fumes box that connects the furnace tube to the fumestubes.

[0039] By means of the invention, recombustion is carried out underoptimum conditions insofar as the flow of the fumes to be processed ishomogeneous in temperature and concentration, and as it is possible toinject the recombustion fuel and the postcombustion air under optimumconditions as regards mixing.

[0040] Furthermore, this recombustion is carried out without any riskfor the furnace tube, which is a sensitive part of the boiler, and thecontainment means (a shell for example) form a screen that protects saidfurnace tube from possible corrosion and/or carbon deposition risks.Finally, the furnace tube is never in contact with reducing gases and/orgases containing substances likely to create carbon deposits.

[0041] Furthermore, the presence of a pilot burner eliminates thepossibility of having large amounts of unburnt fuel at the recombustionzone outlet and the resulting possible explosion risks in the downstreamparts of the boiler.

[0042] It is also possible to provide certain inner parts of thecontainment means with insulating materials in order to adjust thethermal profile according to the recombustion requirements. Thiscontainment means can also be innerly coated with specific materialssuch as certain ceramics which limit the formation of coke.

[0043] The possible coking risks in the recombustion zone are also muchmore limited on the hot walls of the containment means (typicallybetween 800 and 1100° C.) than on walls provided with membranes such asthose commonly used or in the furnace tube, where the temperatures aremost often limited between 250 and 400° C.

[0044] Furthermore, the presence of two fuel injection points in thefurnace tube, a first point close to the main burner and a second pointclose to the recombustion zone, facilitates control of the heatextraction in said furnace tube, in particular under reduced operatingconditions, in order to prevent too great a temperature decrease wherepostcombustion is to be carried out.

[0045] The solution provided advantageously allows a recombustionoperation to be carried out with heavy petroleum products, which wouldcertainly be difficult without containment means because of the foulingrisk.

[0046] The device and/or the method according to the invention alsoallows a strategy consisting in a multistage recombustion fuel delivery,which is certainly more suitable for fuels having a certain amount ofconstitutional nitrogen in order to prevent the conversion of saidconstitutional nitrogen to NO_(x).

[0047] It is also possible to combine a non-catalytic selectivereduction operation with the recombustion operation under optimumconditions for both operations. Such a combination allows to obtainefficiencies that may exceed 90%.

[0048] According to the invention, the maintenance operations in therecombustion zone are very simple insofar as the containment means canbe readily removed from the furnace tube.

[0049] The containment means of the invention have a low mass inrelation to the mass of the boiler and they therefore do not bring asignificant additional inertia. The start times, operating mode changeor holding times are therefore not penalized.

[0050] Advantageously, the solution provided in the present applicationto significantly reduce the NO_(x) emissions level in the fumes finallydischarged can also be applied to boilers equipped with verticalcylindrical furnaces, whether flash boilers or not. It is alsoapplicable to water-tube boilers having cylindrical or parallelepipedicfurnaces.

BRIEF DESCRIPTION OF THE SOLE FIGURE

[0051] Other features and advantages of the present device will be clearfrom reading the description hereafter of a non limitative embodiment ofthe invention, with reference to the sole FIGURE 1 whichdiagrammatically shows a boiler according to the invention.

DETAILED DESCRIPTION

[0052]FIG. 1 shows an industrial two-pass fumes-tube boiler including arecombustion device according to the invention but, of course, theinvention is not restricted to this type of boiler configuration.

[0053] The boiler comprises a burner 1, a cylindrical furnace tube 2,fumes tubes 3 used as recovery means for the heat of the fumes resultingfrom combustion, a cylindrical boiler barrel 4 wherein the water to beheated and vaporized is contained, fumes boxes 5 and 6, a smokestack 7,a water inlet 101, a steam outlet 102 and a device 103 intended forwater level control in said boiler barrel 4.

[0054] Burner 1 is supplied, through line 8, with a gaseous or liquidfuel and with an oxidizer, here in form of a gas which may be air,through line 9. This burner is placed in a quarl 10 and it produces aflame 29 which develops in furnace tube 2 of substantially cylindricalshape and heats the water present around this furnace tube.

[0055] Furnace tube 2 is designed in such a way that, at full power, theflame does not occupy the total length thereof and leaves, in thedownstream part in relation to the direction of the gaseous flow, a freespace that corresponds for example to a third of the total volume ofsaid furnace tube.

[0056] The layout described above is conventionally used in the priorart in the case of a two-pass industrial boiler (Techniques del'ingénieur, B E2, B1480-5 (1998)).

[0057] According to the invention, containment means, in form of asubstantially cylindrical shell 11, are arranged in the downstream partof furnace tube 2. This shell is the place where the fumes coming fromburner 1 will flow and where recombustion and possibly postcombustion ofsaid fumes is carried out. The outside diameter of the shell is slightlysmaller than the inside diameter of the furnace tube so as to create apassage 12 between said shell and said furnace tube. This passage has tobe minimized by means of any known technique so as to allow a minorfraction of the fumes to flow therethrough, but it has to be sufficientto allow the shell to be taken off through a trap door 24. The sectionof passage 12 represents 0.1 to 10% of the total section of flow of saidfurnace tube, preferably 2 to 5%. Said shell can be simply laid in thefurnace tube on support elements 13, but other fastening means known tothe man skilled in the art are also possible.

[0058] Several fuel and oxidizer injection means are arranged insideshell 11, preferably at the center thereof. These means can beconcentric as shown in FIG. 1, with a first central pipe 14 comprising,at the end 15 thereof, mechanical or pneumatic spraying means forinjecting recombustion fuel 30 delivered through a line 16, then, in thedirection of the periphery, a concentric pipe 17 supplied with fuel andoxidizer through lines 18 and 19, used to create at the end 20 thereofan annular pilot flame 31, either multipoint or single, and finally alast concentric pipe 21 supplied with air through a line 22 and allowingto inject postcombustion air 32 introduced through calibrated orifices23 for example.

[0059] These fuel and oxidizer injection means are for example fastenedto a trap door 24. They can be dimensioned as it is known in the art sothat they are cooled only by means of the fuel, the oxidizer andpossibly the carrier fluids they convey, but they can also be cooled byan auxiliary fluid circulating in jackets (not shown in FIG. 1).

[0060] Shell 11 can possibly comprise, in the upstream part thereof (inrelation to the direction of flow of the fumes), means 25 intended tomodify the velocity profile of the fumes at the inlet of said shell.These means include, for example, a grate or a perforated plate. Thegeometry of these means is defined by the man skilled in the art so asto obtain, in combination with the means used for injecting therecombustion fuel, very fast and very homogeneous dispersion of saidrecombustion fuel in the fumes stream to be processed. Means 25 can alsoserve as a thermal screen and protect the recombustion fuel andpostcombustion air injection means from too great a radiation of theflame. Means 25 can also be used to homogenize the temperature in shell11.

[0061] Pilot flame 31 is positioned between recombustion fuel injectionpoint 30 and postcombustion air injection point 32, and preferablysubstantially halfway between these two points.

[0062] Shell 11 is provided with orifices 26 through which the processedfumes flow out prior to entering fumes box 5, then fumes tubes 3.

[0063] According to another embodiment, shell 11 can be entirely open inthe downstream part thereof (which opens into fumes box 5).

[0064] Without departing from the scope of the invention, it is alsopossible for the shell to house only the recombustion zone, thepostcombustion operation being carried out in the fumes box and thepostcombustion air being introduced from the walls of said fumes box,and not from a central pipe.

[0065] The fumes box can have refractory walls or it can be provided,partly or totally, with exchanger tubes 27 connected or not to boilerbarrel 4 according to an embodiment similar or equivalent to theembodiment described in <<Les techniques de l'ingénieur, BE2,B1480-7 >>. On the rear part 28 of the boiler, the exchanger tubes canbe partly off-center in order to leave a free passage for the shell sothat it can be readily removed from the boiler if necessary.

[0066] Shell 11 consists of refractory metal materials. It can beinnerly coated, partly or totally, with insulating materials in order toreduce thermal exchanges with the furnace tube. For example, all or partof the shell corresponding to the recombustion zone can be coated with athin layer of a highly insulating material such as ceramics for example,whereas the part of the shell wherein postcombustion takes place remainssubstantially insulant-free. The insulating materials are deposited onthe walls as it is known in the art, while taking in particular accountof the expansion differences between metallic parts and ceramics.

[0067] The refractory coating of the recombustion zone can also beselected so as to limit the formation of coke, in particular when heavypetroleum fuels likely to generate large amounts of unburnt residues areused.

[0068] When nitrogen-containing fuels are used as recombustion fuel,said recombustion fuel is preferably injected in two (or more) stages: afirst injection immediately at the shell inlet, and a second injectionapproximately halfway between the first recombustion fuel delivery pointand the postcombustion air injection point. The flow rate of therecombustion fuel injected at the first point is calculated so as toconsume all of the residual oxygen from the main combustion zone,without creating a really fuel-rich zone. The second injection is on thecontrary aimed to create a really fuel-rich zone in the second part ofthe recombustion zone.

[0069] Burner 1 uses, for example, natural gas or heavy fuel oil, orpetroleum residues, or any type of fuel used by industrial fume-tubeboilers. It is generally a conventional burner which generates a compactflame and with which it is difficult to develop nitrogen oxidesreduction strategies in the burner. In fact, the most commonly usedfurnace tubes are too narrow to receive low nitrogen oxides emissionburners because they generate most often very developed flames.

[0070] Burner 1 can have a means for driving the oxidizer gas intopartial or total rotation (not shown in FIG. 1) so as to have fumescirculation currents at the furnace tube outlet, rather localized in theneighbourhood of the wall of said furnace tube, and thus to facilitatethe flow of a minor part of the fumes in space 12 in the direction shownin FIG. 1.

[0071] According to another embodiment and operating mode, burner 1 andthe oxidizer injection means can be designed to favour a great axialimpetus of the oxidizer so as to create recirculation currents along thewalls of the furnace tube. Under these conditions, the direction ofcirculation of the fumes in space 12 is opposite to the direction shownby arrows 33 in FIG. 1. A fraction of the fumes present in fumes box 5could thus be recycled upstream from shell 11. The interest of thisoperating mode is that the whole of the fumes can be processed byrecombustion, whereas in the mode mentioned above (burner with means fordriving the oxidizer in a rotating motion), the fraction of the fumescirculating in space 12 is not subjected to recombustion.

[0072] The excess air at the burner in relation to stoichiometry isadjusted so as to typically range between 5 and 25%.

[0073] The position of shell 11 in furnace tube 2 is determined in sucha way that the temperature of the fumes at the inlet of said shell undernominal running conditions ranges between 1100 and 800° C., preferablybetween 1000 and 900° C. The amount of recombustion fuel fed into theshell ranges between 5 and 30% of the total fuel consumed by the boiler,preferably between 10 and 15%. The fuel used by the pilot flametypically consumes only 1% of the total fuel. The flow rate of thepostcombustion air is calculated in such a way that the excess air atthe shell outlet ranges between 5 and 25%.

[0074] The assembly for mixing the recombustion fuel with the fumes tobe processed, consisting of means 25 and of injection device 15, isdimensioned by means of any technique known to the man skilled in theart so that said mixture is obtained in less than 100 ms. In the case ofa gaseous recombustion fuel, injection can be carried out from a singlehead provided with a sufficient number of orifices, as shown in FIG. 1,but other injection modes are also possible at the level of one or morerings whose diameter is larger than the diameter of pipe 14. In the caseof a liquid recombustion fuel, the injection head is calculated as it isknown in the art in such a way that the grain size distribution and theinitial velocities of the droplets provide complete and homogeneouscovering of the fumes stream to be processed, without contact of the nontotally vaporized droplets with the inner wall of shell 11.

[0075] The residence time of the fumes between the recombustion fuelinjection point and the postcombustion air injection point rangesbetween 100 and 500 ms, preferably between 150 and 200 ms.

[0076] The purpose of pilot flame 31 is to provide combustion of therecombustion fuel if the temperature at the inlet of the recombustionzone drops suddenly due to an operating trouble or to an ill-controlledtransient operation. The function of pilot flame 31 is essentially asafety function and there is generally no question of permanentlymaintaining a recombustion operation wherein the recombustion fuel wouldnot be partly or totally oxidized before the pilot burner. A temperatureprobe, not shown in FIG. 1, is placed on pipe 14, with one or moremeasuring points between end 15 of said pipe 14 and end 20 of pipe 17.According to a procedure example, when the temperature measured at thisor these point(s) is below a set value, for example between 500 and1000°C., preferably between 800 and 900° C., the recombustion operationis immediately stopped.

[0077] The postcombustion air delivery device is calculated as it isknown in the art, in such a way that the mixing time of saidpostcombustion air with the gases from the recombustion zone is lessthan 100 ms. Additional means, not shown in FIG. 1, such as a venturi ora diaphragm, can be arranged before or at the same level as thepostcombustion air injection point(s), so as to favour mixing of saidpostcombustion air with the gases from the recombustion zone.

[0078] The postcombustion air possibly contains additives in form ofreactants such as ammonia or urea, or other compounds with equivalenteffects, in order to add a non-catalytic selective nitrogen oxidesreduction to the postcombustion operation proper.

[0079] The recombustion fuel is fed into the shell once the followingoperations have been carried out:

[0080] Firing and power build-up of main burner 1,

[0081] Firing of pilot flame 31,

[0082] Postcombustion air delivery 32.

[0083] To stop the boiler, the same operations are performed, but in thereverse order.

[0084] During operating variations of the boiler, the device is adjustedso as to maintain a substantially constant temperature after injectionof the recombustion fuel. For example, when the power of the boiler isreduced by half, it is possible, according to a first embodiment, todecrease the fuel flow rates in the main burner and in the recombustionzone in the same proportions. However, this approach has the drawback ofdecreasing the temperature at the shell inlet and therefore in therecombustion zone, with risks of significant decrease in the nitrogenoxides reduction efficiency. A second embodiment of the inventionadvantageously uses a strategy which consists in reducing moresignificantly the flow rate in the vicinity of the main burner and inincreasing the recombustion fuel flow rate, the sum of these two flowrates being identical to the flow rate normally required for partialrunning conditions. This procedure reduces thermal exchanges in theupstream part of the furnace tube. This transient phase management modeadvantageously allows to keep a substantially constant thermal level inthe downstream part of said furnace tube and therefore a substantiallyconstant NO_(x) reduction efficiency.

[0085] According to another embodiment of the invention, the thermalprofile in furnace tube 2 during running variations of the boiler canalso be adjusted by displacing the recombustion fuel and postcombustionair injection assembly along the principal axis of shell 11. Thisprocedure allows to change the amount of heat extracted in the furnacetube.

1) A thermal generator comprising a furnace tube (2) wherein a fuel isburnt, recombustion means (14, 15) allowing to reduce the proportion ofnitrogen oxides present in said fumes, and means (3) for recovering theheat of the fumes resulting from said combustion, characterized in thatit comprises containment means (11) in which said recombustion means arearranged. 2) A thermal generator as claimed in claim 1, whereincontainment means (11) are arranged between furnace tube (2) and heatrecovery means (3). 3) A thermal generator as claimed in claim 1 or 2,wherein said containment means are extractable and fastened to a trapdoor (24). 4) A thermal generator as claimed in any one of claims 1 to3, wherein said containment means include means (25) allowing to modifythe velocity profile of the fumes at the inlet of said containmentmeans. 5) A thermal generator as claimed in any one of claims 1 to 4,wherein a fumes box (5) is arranged between containment means (11) andsaid heat recovery means. 6) A thermal generator as claimed in claim 5,wherein said fumes box comprises tubes (27) intended for heat exchangewith the fumes. 7) A thermal generator as claimed in any one of claims 1to 6, wherein said containment means include a shell (11) in which means(14, 15) for injecting a recombustion fuel and means (17, 20) forgenerating a pilot flame are successively arranged, in the direction ofcirculation of the fumes. 8) A thermal generator as claimed in claim 7,wherein said containment means further include means (21, 23) forinjecting air intended for postcombustion. 9) A thermal generator asclaimed in claim 7 or 8, wherein pilot flame (20) is positionedsubstantially halfway between postcombustion air injection point (23)and recombustion fuel injection point (15). 10) A thermal generator asclaimed in any one of claims 1 to 7, wherein air injection meansallowing postcombustion (21, 23) are arranged downstream from saidshell. 11) A method of limiting nitrogen oxides emissions discharged bya thermal generator, characterized in that the following stages arecarried out: a) burning a major part of the fuel in a combustion zone,b) passing the fumes resulting from said combustion into a containmentzone (11), c) mixing in said containment zone a minor part of the fuelwith said fumes. 12) A method as claimed in claim 11, wherein air isinjected into the fumes resulting from stage c) during a stage d). 13) Amethod as claimed in claim 11 or 12, wherein stage d) is carried out insaid containment zone. 14) A method as claimed in any one of claims 11to 13, wherein about 70% to about 95% of the total fuel mass is burntduring stage a), about 5% to about 30% of the total fuel mass is burntduring stage c) and an amount of air allowing to have excess air inrelation to stoichiometric conditions of about 5% to about 25% isinjected during stage d). 15) A method as claimed in any one of claims11 to 14, wherein the fumes from stage c) are contacted with a pilotflame prior to stage d). 16) A method as claimed in any one of claims 11to 15, wherein a chemical reactant other than the fuel allowingreduction of the nitrogen oxides by non-catalytic selective means isadded to the air injected during stage d). 17) A method as claimed inany one of claims 11 to 16, wherein, when the generator works underreduced operating conditions, the flow rates of the fuel injected in thecombustion zone and in the containment zone are adjusted so as tomaintain a substantially constant temperature in said containment zone.